Ryan Lochte says he "always" pees in the pool, and Michael Phelps thinks "Chlorine kills it so it's not bad." So it's totally safe to let go underwater, right?

Christmas Day, 2006, a healthy six-year-old boy swimming in an indoor pool in Nebraska starts coughing and struggling to breathe. His parents take him out of the water and the coughing fit continues. Then he begins vomiting. Five hours go by and his condition deteriorates.

Swimming Pools Are Gross

A swimmer sweats between .20-1.76 liters into the pool per event.

A swimmer releases 25-177 milliliters of urine into the pool per event.

One out of every five Americans admits to peeing in a public pool.

Ninty three percent of Americans say they would never reuse someone else's bath water, yet water parks receive approximately 360 million visits each year.

Barking with every cough and choking with each breath, the boy is rushed to the emergency room where his condition is finally stabilized, after a dexamethasone injection, a corticosteroid, and three doses of racemic epinephrine (a.k.a. adrenaline). The attending physician records chlorine irritation as the cause of illness.

Over the last decades, a growing body of scientific research suggests that peeing in the pool is not as harmless as we once thought. A soon-to-be-published study in the journal Environmental Science & Technology builds on prior research to show that the binding of chlorine and uric acid, a chemical introduced into pools almost entirely from urine, can create two potentially harmful chemicals, cyanogen chloride and nitrogen trichloramine, says study co-author Ernest Blatchley, a professor at Purdue University. These chemicals can then enter the air inside the swimming pool through evaporation.

Used as a chemical weapons sporadically during WWI, cyanogen chloride interferes with the body's ability to use oxygen and can harm the central nervous system, lungs, and heart. Trichloramine is associated with asthma-like symptoms, and abundant levels of the chemical have been linked with swimming pool public health outbreaks, such as the one that affected 665 people at an indoor waterpark in 2007.

The relationships between disinfection by-products and health problems commonly associated with swimmers, lifeguards, and pool staff, remains unclear. But breathing in air at an indoor pool puts employees at "an excess risk for respiratory symptoms indicative of asthma" according to one European study. Another study from 1994 found chloroform in the blood of competitive swimmers who frequented an indoor pool, (but not in outdoor pool swimmers). In the bloodstream, chloroform can damage your liver and kidneys over a long period of exposure.

So should you stop going to the pool? No, says Michele Hlavsa, the Chief of the Center for Disease Control's Healthy Swimming Program. The health benefits still outweigh the possible dangers. But be wary of indoor pools that don't look well maintained. "A good, healthy pool doesn't smell at all." Hlavsa says.

Without proper maintenance or effective ventilation systems, these hazardous gases will build-up. So look for indoor pools with open access—through windows and doors—to the oudoors and that use fans to boost airflow over the pool’s surface. And don’t underestimate the importance of showering before you enter the pool and actually ignoring the impulse to pee. As Blatchley and his co-authors write, it is after all "a voluntary action for most swimmers."

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